Process of dimerizing carboxylic acids in a corona discharge



United States Patent 3,356,602 PROCESS OF DHMERIZING CARBOXYLIC ACIDS IN A CORONA DISCHARGE John A. Coilman, Ballston Spa, and William R. Browne, Scotia, N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Filed Mar. 26, 1965, Ser. No. 443,159 6 Claims. (Cl. 204165) The invention relates to a process of treating carboxylic acids and esters thereof within a corona discharge.

It has long been recognized that gaseous electrical discharges are capable of effecting chemical alterations. When two bare, spaced electrodes are subjected to a large potential difference within the atmosphere, the portion of the atmosphere lying between the electrodes is rapidly ionized so that an electrically conductive path is provided between the electrodes. The resulting high current, low voltage electrical discharge is an arc and is readily identified visually by its limited areal extent and sharply defined boundaries. An electric arc is chemically highly disruptive and for this reason unsuited for treating organic materials except Where a high degree of fragmentation is desired.

The terms silent electric discharge and voltolization refer to high voltage, low current electrical discharge phenomena clearly distinguishable from an electric are by being soft and diffused. Reference to silent electric discharge or voltolization conducted at pressures substantially beneath atmospheric is indicative of glow discharge, which is a bare electrode phenomenon sustainable with alternating or direct current. The electrodes are maintained bare to produce contact ionization of the surrounding atmosphere. In order to prevent ionization from bridging the electrodesi.e., generating an arcit is necessary that the atmospheric pressure be reduced. The pressure reduction greatly increases the mean-free path of chemically active ions and electrons between collisions thereby increasing the mean particle velocity. Such properties are recognized, for example, in Reactions of Hydrocarbons in Electrical Discharges, Chemical Reviews, vol. 28.

A corona discharge, which is visually similar to a glow discharge, differs markedly in chemical and electrical characteristics. A corona discharge is a capacitative phenomenon requiring at least one dielectric barrier between spaced electrodes and an alternating current. Chemically active ions and electrons are formed by capacitive excitation of the atmosphere rather than by contact ionization as in a glow discharge. The presence of a dielectric barrier to eliminate arcing obviates the necessity of pressure reduction. Atmospheric or higher pressures produce a shorter mean-free path between particle collisions and reduced mean particle velocities. Accordingly, chemical treatments by corona discharge are substantially less disruptive in character than those obtainable by either electric are or glow discharge treatments.

It is an object of our invention to provide a process of treating carboxylic acids and esters in an electric discharge and a non-oxidative atmosphere to achieve a useful net change in the materials without decarboxylation, without introducing oxidative color impurities, and without complete elimination of any unsaturation which may be present. In the case of monoesters and unsaturated polyesters, it is an object to achieve a net dimerization of the material. It is a further object to treat carboxylic acids in a nonreactive atmosphere.

It is our discovery that useful products may be obtained by subjecting monocarboxylic acids and esters thereof to a corona discharge propagated in a Group 0 inert gas or a reducing atmosphere maintained at or above atmos- 3,356,662 Patented Dec. 5, 1967 ice pheric pressure. Whereas electrical discharge treatments of organic materials have heretofore been considered to yield more or less random chemical associations and cleavages, we have discovered that in corona treatment of monocarboxylic acids and esters certain types of reactions will predominate to produce useful net changes in the materials. When monocarboxylic acids and monoesters are treated in a corona discharge, a net dimerization is observed regardless of whether unsaturation is present or absent. In corona treatment of polyesters of monocarboxylic acids preponderant dimerization is observed when the acid moiety is predominately unsaturated, as in polyunsaturated triesters, while molecular cleavage is predominant in relatively saturated triesters. It is our discovery that corona discharge dimerization may be sufiiciently controlled to retain a substantial degree of original unsaturation. Our process accordingly appears well suited to place liquid polyunsaturated oils in conveniently usable solid form without the impairment of unsaturation required by conventional processes. A notable advantage of our corona treatment process is that net changes can be effected in carboxylic acids and esters without decarboxylation. Further, by using a reducing or Group 0 inert gas atmosphere, low color products substantially devoid of discoloring impurities of generally oxidative origin can be obtained. The atmospheric composition is also of utility in modifying the net chemical change produced by corona treatment. Finally, we have discovered that corona treatment of a monocarboxylic acid or ester can be more efliciently achieved in a packed dielectric bed.

As previously noted, a corona discharge is a soft, dif fused visual display, capacitative in nature, requiring for propagation at least one dielectric barrier between spaced electrodes connected to a source of alternating current. When dielectric barriers are employed adjacent each of two spaced electrodes in achieving a corona discharge, the discharge phenomenon is frequently termed an electrodeless discharge, whereas when a single dielectric barrier is employed to insulate a single electrode, the resulting phenomenon is frequently termed a semicorona discharge. Both electrodeless and semicorona discharges are included within the scope of the invention and are included by the generic term corona discharge.

A corona discharge is a gas phase phenomenon. When a gas is placed in an electric field generated by spaced electrodes connected to a source of alternating current, the gas absorbs energy from the electric field. The energy absorption may result in activating electrons into higher energy orbitals, in dissociating diatomic gases into free radicals, in forming gaseous ions, or in any combination of these. Further, since the electric field of an alternating current cyclically decays, the absorbed energy is cyclicaily liberated by the gases.

When it is desired to treat a normally liquid material such as a carboxylic acid or ester. thereof, it is necessary to produce the corona discharge in an atmosphere in contact with at least one surface of the liquid to be treated. The exposed surface to be treated may be a static surface as, for example, the surface of a liquid confined in a beaker. More efiicient treatments are achieved in dynamic systems where the material to be treated is flowed through the corona zone in the form of a curtain or film. We have discovered that highly efiicient treatments may also be provided by flowing a liquid to be treated through a packed bed of dielectric particles. Eflicient treatments may ordinarily be obtained using packed beds having particles ranging from as low as 0.07 mm. in longest dimension. The packed bed serves to greatly multiply the surface area of the material in contact with the corona and to improve distribution of the corona. Any solidified ma-v terial produced within the packed bed during corona treatment can be easily removed by contemporaneous or subsequent heating of the packed bed by conventional means.

It is our discovery that particular advantages may be achieved in treating a carboxylic acid or ester in a reducing or a Group inert gas atmosphere. By treating a carboxylic acid or ester in a reducing atmosphere, any tendency toward oX-idative degradation of the organic molecule can be obviated. Hydrogen constitutes apreferred reducing atmosphere, since its relatively simple structure allows it to be easily activated to generate a corona discharge. Also, the hydrogen atmosphere is capable of interacting with the organically bound hydrogen atoms without generating elemental impurities.

Group 0 inert gases, having minimal energy absorbing capabilities, are even more easily activated to generate a corona discharge than hydrogen. It has been observed that Group 0 inert gases are capable of generating a corona discharge at lower voltages than other gases and thata more pronounced and uniform corona is generated at equivalent voltages. It is contemplated that Group 0 inert gases may be employed in combination with other gases, if desired. Our invention includes the use of one or more Group 0 inert gases in any proportion with gases of known utility in treating oils and waxes with a silent electric discharge, such as nitrogen, carbon monoxide, carbon dioxide, etc., or with a reducing atmosphere such as hydrogen. Group 0 inert gas mixtures with air are contemplated in all proportions above the average proportion of Group 0 inert gas in air, which is 0.942 percent by volume.

It. is a further procedural advantage of our invention to treat a carboxylic acid or ester with a corona discharge propagated'in a gaseous medium maintained at or above atmospheric pressure. Inasmuch as corona discharge is maintained between spaced electrodes and arcing prevented by the dielectric properties of the barrier material rather than by the atmosphere density, as is the case in glow discharge, a corona discharge may be maintained at pressures up to atmospheres or higher. Operation at ambient or positive pressures allows elimination of expensive and cumbersome vacuum producing equipment. Additionally, the explosion hazard due to air leakage into a corona propagating reducing atmosphere such. as hydrogen, for example, is avoided.

A. corona discharge can be generated using only an alternating current. Generally, high frequencies are preferred since the phenomenon is capacitive in nature. Frequencies ranging from c.p.s. to 500,000 c.p.s. are contemplated. A preferred frequency range is from 3,000 c.p.s. to 10,000 c.p.s. The current, voltage, and power utilized in generating a corona discharge for a specific process will vary over wide limits depending on the thickness of the dielectric barrier or barriers employed, the electrode spacing, and the nature of the gaseous media lying within the discharge area. It is generally preferred that the corona producing gap between spaced electrodes be no more than /2 inch. Preferred total barrier thicknesses range from /8l1'1Ch to 4 inch, irrespective of whether one or two barriers are employed. In a hydrogen atmosphere, with a A: inch quartz barrier, an exemplary desired voltage range extends from 8 to 16 kv. peak.

Materials to be treated with a corona discharge according to our invention include monocarboxylic acids, preferably those having from 2 to 30 carbon atoms and most preferably those having 10 to carbon atoms. The acids may be saturated or unsaturated. Preferred acids are those having up to three ethylenic linkages. Exemplary saturated carboxylic acids include butyric, isovaleric, caproic, caprylic, capric, lauric, myristic, palrnitic, stearic, arachidic, behenic, lignoceric, cerotic, etc. Exemplary unsaturated carboxylic acids include A -decylenic, stillingic, A -dodecylenic, palmitoleic, oleic, ricinoleic, petroselinic, vaccenic, linoleic, linolenic, eleostearic, licanic, parinaric, gadoleic, arachidonic, cetoleic, erucic,

selacholeic, hydnocarpie, chaulmoogric, gorlic, mycolipenic, mycoceranic, etc., as well as stereoisomers thereof and other similar acids. If desired, acids having acetylenic as well as ethylenic unsaturation may be treated. Included ages, since less opportunity for energy absorption through resonance is afforded.

Our invention may additionally be practiced with esters of acids of the type above noted, preferably with those having alcohol moieties of less than six carbon atoms. The term alcohol as herein employed designates hydrocarbon derivatives in which one .or more hydrogen atoms are replaced by hydroxyl groups. Suitable alcohols include primary, secondary, and tertiary monohydric alcohols as well as dihydric alcohols, trihydric alcohols, etc. Suitable esters include those capable of yielding upon hydrolysis such exemplary alcohols as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, allyl alcohol, n-butyl alcohol, isobutyl alcohol, sec-butyl alcohol, t-butyl alcohol, n-amyl alcohol, isoamyl alcohol, t-amyl alcohol, neo-pentyl alcohol ethylene glycol, propylene glycols, butylene glycols, glycerol, erythritol, pentaerythritol, etc. Esters of particular interest are fats and oils including, for example, coconut oil, babassu oil, palm kernel oil, palm oil, olive oil, castor oil, peanut oil, rape seed oil, lard, whale blubber, corn oil, satflower oil, cottonseed oil, soybean oil, triolein, trilinolein, trilinolenin, tristearin, tricaprin, trilaurin, trimyristin, tripalmitin, etc.

The materials to be treated in a corona discharge may be mixtures of carboxylic acids and/or esters thereof. The materials may be derived from natural or synthetic sources. The energy to be expended on the materials will depend on the particular material to be treated together with the net structural rearrangement desired. Generally, treatments in the range of from 0.01 to 10 watt-hours/ gram are effective to produce substantial net changes in the material, with treatments in the range of 0.1 to 5 watt hours/gram being preferred.

Carboxylic acids as; well as unsaturated monoesters thereof subjected to corona discharge according to the present invention show visibly discernible viscosity increase. Liquids may be solidified by treatment. Tests of materials treated indicate net dimerization and absence of decarboxylation. Unsaturated materials retain unsaturation subsequent to treatment. In the case of substantially saturated fatty esters, the rate of molecular cleavage is believed to exceed the rate of dimerization so that a net loss of molecular weight is exhibited. It is believed, however, that both useful dimers and cleavage products may be obtained through conventional chemical separation techniques.

Our invention may be better understood by reference to the following examples which are intended to illustrate rather than limit the invention.

Example 1 Thirty grams of safflower oil is placed in a glass beaker. The beaker is floated on a pool of mercury in a closeable reaction vessel. An electrical connection is made to the pool of mercury so that it may function as a ground electrode. A doughnut shaped high tension electrode consisting of quartz vessel .filled with mercury is lowered into the beaker so that it is 4 mm. above the surface of the oil. The reaction vessel is closed and purged with hydrogen. A static hydrogen atmosphere is maintained at a pressure of 1.08 atmospheres. The beaker forms one dielectric barrier of 1 inch thickness and the quartz vessel forms a second dielectric barrier of inch thickness.-An alternating current having a frequency of 10,000 c.p.s. is connected to the ground electrode and to the high tension electrode. A voltage ranging from 10.1 to

11.8 kv. peak is impressed across the electrodes for a period of 80 minutes. A corona power of 7 watts is employed. A temperature of 50 C. is maintained within the reaction vessel. A corona discharge is generated in value. The weight percentage acid dimer in the treated methyl linoleate samples is determined by the formula 100M (A IM Weight percent acid dimer-W contact with the upper surface of the oil. The saffiower 5 oil is visually inspected before and after corona treat- Where ment. The results are set out in Table I. The hydrogen atmosphere is examined by mass spectrometry and found Mlzmolecular welght of monomer, free of ester cleavage products. Acid number tests of the 2= f Weight of l and ester indicate no decarboxylatiom 1O M=expenmentally determined average molecular weight.

Example 2 The test results are set out in Table II. The process of Example 1 is repeated substituting oleic TABLE II acid for safflower oil, except that a voltage of 11.2 to E a 4 13.9 kv peak is employed for a period of 60 mlnutes. Sample l f g Average MW Dimer The oleic acid 15 visually mspected before and after co- N0. hrs/g Increase -P rona treatment. The results are set out in Table I. Freedom from cleavage products and decarboxylation is de- 1 1 3&1 17 11.0 termined as in the case of safilower oil. 20 :11:11: 2 3;}, 3g $21 Example 3 The process of Example 1 is repeated substituting lino- Examples leic acid for safliower oil, except that a voltage of 15.1 Five 100 grams samples, two of oleic acid and three to 16.8 kv. peak is employed for a period of 87 minutes 25 of linoleic acid, are successively treated in a corona with a corona power of 10 watts. The reaction temperareactor of the type described in Example 4. The reactor ture is maintained at 70 C. The linoleic acid is visually in each instance is first purged with argon and then a flow inspected before and after corona treatment. The results of 4.6 cc./ min. hydrogen is circulated through the reactor are set out in Table I. Acid number tests indicate no deat slightly above atmospheric pressure. The upper eleccarboxylation while mass spectrometry of the hydrogen trode is maintained Mt inch above the upper surface of atmosphere yields only traces of methane. the sample, and the sample is maintained at approximately TABLE I Energy Input Appearance Material (watthr./gm.)

Before After Satfiower oil. 0. 21 Oil, faintly yellow Colorless oil, white solid. Oleic acid". 0.22 Yellow oil Slushy solid. Linoleic aeid 0.49 ...do Do.

Example 4 C. during corona treatment. A corona frequency of A beaker containing 50 grams of methyl linoleate is placed in a corona reactor of the type shown in FIGURE 1 of the commonly assigned application of Browne et al., Ser. No. 409,199, filed Nov. 5, 1964. The reactor is equipped with two electrodes, the upper of which is provided with a ,3 inch quartz barrier. The bottom of the beaker provides a second A quartz barrier adjacent the bare lower electrode. The reactor is first purged with argon and then a flow of 4.6 cc./min. hydrogen is circulated through the reactor at slightly above atmospheric pressure. The reactor is operated at a frequency of 10,000 c.p.s. and voltages of approximately 20 kv. peak. The upper electrode is maintained A inch above the upper surface of the ester. The methyl linoleate is maintained at approximately 50 C. during corona treatment.

The methyl linoleate is subjected to a corona discharge propagated in the hydrogen atmosphere. After a treatment of 1 watt-hour/ gram, a IO-gram sample is removed with a pipette and labeled Sample A. The remainder is again treated with a corona discharge until a total treatment of 2 watt-hrs./ gram is attained. Twenty grams of the remainder is labeled Sample B. The finally remaining twenty grams of methyl linoleate is further treated with a corona discharge until a total treatment of 4 watt-hours/ gram is attained. The finally treated twenty grams is labeled Sample C.

Samples A, B, and C as well as an untreated sample of methyl linoleate are subjected to a molecular weight determination using a vapor pressure osmometer and benzene as a solvent. The accuracy of the test is corroborated by the fact that the molecular weight of untreated methyl linoleate is found to be 294, the theoretical 10,000 c.p.s. is employed. The voltage, wattage, energy input, and dimerization are indicated in Table III. Acid number tests of each sample indicated no decarboxylation.

Example 10 The procedure of Examples 5-9 inclusive is repeated except that a -gram sample of stearic acid is used. Since stearic acid is normally a solid at room temperature, the acid is heated to approximately 70 C. initially preceding corona treatment. Suflicient heat is generated in the corona treatment to maintain the acid in the liquid form. The test results are set out in Table III.

TABLE III Example Power Voltage Energy Input Dimerization No. Watts KVP (watbhrs/ (Wt.-

gm.) percent) Example 11 A l20-gram sample of linoleic acid is circulated at a rate of 315 grams/ minute through a swirl tube reactor of the type disclosed in FIGURE 1 of the commonly assigned application of Dibelius et al., Ser. No. 411,192, filed Nov. 16, 1964. Hydrogen is circulated through the reactor at slightly greater than atmospheric pressure and at a rate of 5.3 cc./min. No exterior electrode cooling is Example 12 A 125-gram sample of linoleic acid is treated for 110 minutes at 42 C. according to the procedure of Example 11. The total energy input is 4.12 watt-hrs./gram. At the end of the treatment period, the sample is noted to be semisolid at 25 C.

Example 13 The procedure of Example 11 is repeated except that a voltage of approximately 16 kv. peak is applied at a reactor temperature of 51 C. The corona is propagated for 33 minutes. During the latter portion of the run, a gel forms on the inner wall of the reactor. The treated linoleic acid is a semisolid at 25 C. An energy input of 2.06

watt-hrs./ gram is supplied.

Example 14 The swirl reactor employed in Example 11 is plugged below the corona generating zone with glass wool. The corona generating zone is filled with 8 to 12 mesh alumina. A 120-gram sample of linoleic acid is circulated through the reactor at a rate 18.5 grams/minute while hydrogen is circulated through the reactor at a rate of 4.8 cc./ minute. The inner electrode is water cooled so that it is maintained at a temperature of 47 C.

The corona reactor is operated at a voltage of approximately 19 kv. peak and a frequency of 10,000 c.p.s. The power density, based on the area of the inner electrode, is 5.6 watts/m At the end of 38 minutes, with an energy input of 1.24 watts/hrs./gram, the linoleic acid gels within the packed bed.

Example 15 linoleic acid at a power input ranging from 7.8 to 13.6,

watts. The linoleic acid remained a liquid although some solidification was noted around the edge of the petri dish. Also some solidified material was noted to have diffused onto the upper electrode. The material exhibited an iodine number of 122.3 after treatment as opposed to an initial iodine number of 119.7. The molecular weight increased from an initial 280 to 305 indicating 16.4 percent dimerization.

8 Examples 16 and 17 The procedure of Example 15 is repeated using peanut oil in place of linoleic acid. A first run is conducted at a. voltage ranging from 16.9 to 17.6 kv. peak and a wattage ranging from 7.6 to 10.9 watts for sufficient time to yield an energy input of 1.15 watt-hr./ gram. An iodine value: of 92.0 is noted as opposed to an initial iodine value of 96.4 while the average molecular weight is determined to be 712 as opposed to an initial molecular weight of 963..

A second run is conducted substituting hydrogen for argon. A voltage ranging from 28 to 31.9 kv. peak and a wattage of from 26 to 34 is employed. A total energy input of 1.14 watt-hrs./ gram is provided. The final average molecular weight is noted to be 886 while the iodine number is noted to be 92.1.

While we have described our invention in terms of certain illustrative examples, it is apparent that numerous modifications will be readily suggested to those skilled in the art. For this reason, it is intended that the scope of the invention be determined with reference to the following claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A process of corona discharge treatment comprising placing in an atmosphere containing a Group 0 inert gas maintained at a pressure of at least one atmosphere a material chosen from the group consisting of a monocarboxylic acid having from O to 3 ethylenic linkages and from 2 to 30 carbon atoms and an ester of said monocarboxylic acid and an alcohol having from 1 to 6 carbon atoms, and

subjecting the material to a corona discharge supplied with electrical energy in the amount of 0.01 to 10 watt-hrs./ gram.

2. A process according to claim 1 in which the material is subjected to 0.1 to 5 watt-hrs./ gram.

3. A process according to claim 1 in which the Group 0 inert gas is continuously circulated through the corona discharge.

4. A process according to claim 1 in which the Group 0 inert gas is present in a concentration in excess of 0.942 percent by volume.

5. A process according to claim 1 in which the material is continuously circulated through the corona discharge.

6. A process according to claim 1 in which the corona discharge is propagated in a packed bed of particulate dielectric material.

References Cited UNITED STATES PATENTS 1,578,624 3/1926 Anderson 204-167 1,621,143 3/1927 Vogel 204167 1,964,891 7/1934 Pier et al. 204-468 2,147,177 2/1939 Seto et al 204-167 2,161,987 6/1939 Tilton 204-168 2,167,726 8/1939 Richardson 260-407 2,170,665 8/1939 Russell 260 -407 ROBERT K. MIHALEK, Primary Examiner. 

1. A PROCESS OF CORONA DISHCARGE TREATMENT COMPRISING PLACING IN AN ATMOSPHERE CONTAINING A GROUP O INERT GAS MAINTAINED AT A PRESSURE OF AT LEAST ONE ATMOSPHERE A MATERIAL CHOSED FROM THE GROUP CONSISTING OF A MONOCHARBOXYLIC ACID HAVING FROM 0 TO 3 ETHYLENIC LINKAGES AND FROM2 TO 30 CARBON ATOMS AND AN ESTER OF SAID MONOCARBOXYLIC ACID AND AN ALCOHOL HAVING FROM1 TO 6 CARBON ATOMS, AND SUBJECTING THEMATERIAL TO A CORONA DISCHARGE SUPPLIED WITH ELECTRICAL ENERY IN THE AMOUNT OF 0.01 TO 10 WATT-HR./GRAM. 